Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
In a semiconductor fabrication facility (fab), metal film quality can be monitored by a sheet resistance (Rs) measurement tool. Both film resistivity (ρ) and thickness (THK) information are represented by the Rs value according to the formula Rs=ρ/THK. However, process engineers still need the separate value for film thickness and/or resistivity for some cases. For example, during chemical mechanical planarization (CMP) steps, film thickness is needed for CMP rate estimation. When the copper film is polished to a different thickness, its resistivity will change due to a surface scattering effect. If a constant resistivity value is used to calculate the film thickness, the calculated thickness will deviate from the real value, which causes an inaccurate estimation of CMP rate. Besides the thickness effect, grain size variation of copper electrochemical plating (ECP) conditions, like growth temperature or plating rate, also may change the resistivity, which makes thickness estimation directly from Rs value even more difficult.
Metal self-annealing is another example with difficult Rs measurements. Grains in a metal film tend to grow after the metal film is deposited on a substrate, which reduces its resistivity over time. This gradual Rs trend with time adds uncertainty in process control.
Usually resistivity is calculated from Rs and film thickness. Several techniques have been used to measure metal film thickness directly in a semiconductor fabrication facility (fab). These techniques include TEM/SEM, XRD/XRF, profiler tools, spectroscopic ellipsometry (SE) tools, and optical-acoustic techniques.
For TEM/SEM, a wafer needs to be broken into small pieces. Then a cross-section image can be obtained at the edge of test piece. TEM/SEM equipment can be expensive and sample preparation time is long. Furthermore, this kind of measurement cannot be used as a routine fab metrology method because it is slow and destructive.
For XRD/XRF, the metal film thickness can be determined by scattering strength, interference fringes, fringe spacing and scattering strength, profile simulation, and other methods. All methods are based on modeling and fitting. Assumptions, simplifications, and reference standards are used for these methods because x-ray signals are related to material properties of a film and/or substrate, such as composition, crystal structure, texture, strain/stress, defects, and/or surface oxide/roughness. This limits XRD measurement applications. XRD/XRF methods also have low throughput, which may be insufficient in a fab. Several expensive XRD/XRF tools may be necessary for signal collecting on a single site.
For a profiler tool, the film thickness is obtained by removing part of a film that was grown on a wafer, which forms a step at the edge of film covered and uncovered regions. A diamond micro pin scans across the edge and the height values are measured. The film thickness is reported as a height difference between a region covered by a film and a region not covered by the film. Extra processes like lithography and etching are needed to provide the two regions, which adds time and expense. Furthermore, the profiler may only be capable of detecting height differences greater than 1 nm.
SE is used for dielectric film thickness characterization with optical modeling of parameters including n, k, and film thickness. However, the penetration depth of light, even at the infrared range, is small for a metal film due to the high conductivity of such metal films. As a result, the film thickness measureable range is limited and correlated to metal conductivity. Typically the maximum film thickness is several nanometers. In an instance, the maximum film thickness is <2 nm. These limitations make SE inappropriate for many measurements in a fab.
An optical-acoustic method can report metal film thickness. For example, a femtosecond-laser can achieve a fine spatial resolution down to scale of nanometers. The laser pulse frequency range can be from 0.1 to 1 THz. When the femtosecond-laser is incident on the metal film on the wafer surface, the surface is heated and generates an elastic wave propagating toward the wafer. When the wave propagates through an interface between the metal film and the wafer, waves reflect and return to the wafer surface, which causes a deformation at the wafer surface. Another probing laser monitors the surface deformation (reflectance signal) with time. The film thickness is obtained by fitting the time-resolved reflectance signal. The time interval between two reflectance peaks is determined from the fitting curve. The film thickness can be calculated from the speed of sound. The speed of sound inside the metal is set as a constant from a reference or calibration. Optical-acoustic techniques have shortcomings that make it inadequate in a fab. First, it has a poor typical stability (1sigma:0.5%) as compared to other methods. Second, there is an error source from the assumption of constant speed of sound. The speed of sound is related to material properties (e.g., density, bulk modulus, and Poisson's ratio). These properties may change with different process conditions. For example, metal grain size will change the modulus. Sometime 5% error can occur when Co thickness is measured, which is not acceptable for fab process control. Third, the film thickness may only be accurate from 40 A to 8 μm.
Therefore, improved methods and systems to measure film thickness are needed.